Wafer surface inspection apparatus and wafer surface inspection method

ABSTRACT

A wafer surface inspection method and apparatus of high sensitivity, and free from performance degradation in terms of cleanliness, coordinate repeatability of foreign particles and the like. Gas for cooling is sprayed onto a laser irradiation position on the wafer surface to prevent an increase in temperature of the foreign particles and to suppress break-down of the foreign particles.

BACKGROUND OF THE INVENTION

The present invention relates to a surface inspection apparatus and asurface inspection method of a subject to be inspected, and relates forexample to a wafer surface inspection apparatus and a wafer surfaceinspection method for inspecting for foreign particles or foreignmatter, defects and the like on a semiconductor wafer surface in amanufacturing process of a semiconductor device.

In a manufacturing process of a semiconductor device, a circuit isformed by transferring a pattern onto a bare wafer and etching the same.In the manufacturing processes of various semiconductor devices duringwhich circuits are formed, foreign particles adherent to the wafersurface, defects and the like become major factors causing a decrease inyield. The foreign particles adherent to the wafer surface and thedefects are managed in each manufacturing process, and a wafer surfaceinspection apparatus detects the foreign particles adherent to the barewafer surface, the defects present on the wafer surface and the likewith high sensitivity and with high throughput.

The methods of inspecting for foreign particles and defects on the waferare largely classified into those using a charged particle beam such asan electron beam or the like and those using light. The methods usinglight include one that uses a camera to take an image of the wafersurface to analyze the image information, and one that uses a lightreceiving element like a photomultiplier tube to detect scattered lighton the wafer surface to analyze the degree of scattering of light. Forexample, in a surface inspection apparatus of the type irradiating thesurface of the wafer with a laser beam, the scattered light from foreignparticles caused by irradiation with the laser beam is detected by adetector and is subjected to analog-to-digital conversion to be outputas coordinate data. To increase the throughput of inspection, aninspection table with a work (wafer) mounted thereon is rotated at highspeed, and a stage mounted with the inspection table is scannedhorizontally in an axial direction. As a surface inspection apparatus ofthis type, one described in JP-A-55-102233, for example, is known.

SUMMARY OF THE INVENTION

In the method of irradiating a wafer surface with a laser beam anddetecting scattered light from foreign particles or defects, detectionsensitivity would be improved if the density of energy of laserirradiation is increased. However, it has become found that when theforeign particles are made of organic matter prone to break down, theforeign particles may be broken down (destructed) by heat of the laserirradiation, in which case the particles broken down would fly aroundthe area and adhere to the wafer surface to thereby contaminate thewafer. Conventional inspection apparatuses use laser of low power, andthus, such phenomenon of destruction of foreign particles by the laserbeam did not occur. However, with an increase in sensitivity ofdetection of foreign particles, laser of high power has come to be used,and thus, the phenomenon of destruction of foreign particles by laserirradiation has begun to emerge. Further, in the wafer surfaceinspection apparatus, inspection is carried out while rotating the work(wafer) to be measured, as described above. Thus, the destructedparticles would fly around the area and adhere to the wafer surface in aspiral fashion over a wide area, to contaminate the wafer.

For a wafer surface inspection apparatus, high cleanliness and highprecision coordinate repeatability of foreign particles are required.The wafer contamination attributable to the destruction of foreignparticles, however, would degrade such performance. To address thisproblem, although the density of energy of laser irradiation may bedecreased to avoid destruction of foreign particles, it would lowerdetection sensitivity as well. Accordingly, there is a demand for amethod that can prevent destruction of foreign particles attributable tolaser without lowering the detection sensitivity.

An object of the present invention is to provide a wafer surfaceinspection apparatus that is highly sensitive and free of degradation ofdevice performance in terms of cleanliness, coordinate repeatability offoreign particles and the like.

It is considered that foreign particles are broken down (destructed) byirradiation with a laser beam because the temperature of the particlesincreases due to the irradiation with the laser beam. Thus, by sprayinggas for cooling onto the laser irradiation position on the wafer, theincrease in temperature of the foreign particles can be prevented tosuppress destruction of the foreign particles. The conditions forspraying gas for cooling depend on the density of energy at the point oflaser irradiation. That is, when the density of energy is large, it isnecessary to increase the flow velocity and the flow rate of the gas forcooling to increase the amount of heat radiation from the foreignparticles.

JP-A-55-102233 describes that upon detection of foreign particles on thewafer, air is blown to blow off the particles. However, the foreignparticles generally adhere to the wafer so firmly that they would not beblown off by just blowing the air. Rather, blowing off the foreignparticles would pose another problem that the range of contaminationwould increase. In the present invention, gas of high cleanliness enoughto cool the foreign particles is sprayed at the flow velocity and theflow rate with which the foreign particles would not be blown off.Further, in JP-A-55-102233, the air is blown before analysis, i.e.,before irradiation of light, to blow off the foreign particles. Incontrast, in the present invention, the gas for cooling is sprayed ontothe laser irradiation position at the same time as irradiation of thelaser beam, in order to cool the foreign particles being irradiated withthe laser beam.

Namely, the wafer surface inspection apparatus according to an aspect ofthe present invention includes: a table holding a semiconductor wafer;light irradiation means for irradiating a small area on a surface of thesemiconductor wafer held on the table with a laser beam; light detectionmeans for detecting light scattered from the semiconductor wafer byirradiation with the laser beam; table drive means for rotating andconcurrently linearly moving the table with respect to the lightirradiation means; and gas spraying means for spraying gas onto the areaon the semiconductor wafer irradiated with the laser beam by the lightirradiation means. Preferably, the apparatus has a control unitcontrolling the light irradiation means and the gas spraying means, andthe light irradiation means and the gas spraying means are controlled bythe control unit such that the gas is sprayed from the gas sprayingmeans at least while the beam is irradiated from the light irradiationmeans.

According to the present invention, irradiation of laser and spraying ofgas with respect to the wafer are carried out at the same time and forthe same location. This can increase the density of energy of irradiatedlaser at the limit of not destroying the foreign particles, compared tothe conventional case. Accordingly, it is possible to carry out theinspection without destructing the foreign particles, while preventinglowering of detection sensitivity.

Other objects, features and advantages of the present invention willbecome apparent from the following description of the embodiments of thepresent invention when taken with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic configuration of a wafer surfaceinspection apparatus according to the present invention.

FIG. 2 is a configuration diagram (piping system diagram) of an air blowunit.

FIG. 3A shows that contamination in a spiral form has occurred with aconventional inspection method.

FIG. 3B shows a result of inspection by a method of the presentinvention.

FIG. 4 shows the relation between the laser power and the gas sprayingflow rate having an effect of suppressing destruction of foreignparticles.

FIG. 5 is a diagram (piping system diagram) showing an air blow unitincorporating a mass flow controller.

FIG. 6 shows a setting screen of gas flow rate and a displayed state ofscatter plot.

DESCRIPTION OF THE EMBODIMENTS

The inspection method and the inspection apparatus of the presentinvention are applicable to a flat plate-shaped subject to be inspected,such as a semiconductor wafer, a glass substrate for a liquid crystalpanel, a disk substrate and the like. In the following, embodiments ofthe present invention will be described with reference to the drawings,taking a semiconductor wafer as an example.

FIG. 1 shows a schematic configuration of a foreign particle inspectionapparatus of the present invention incorporating gas spraying means (gasspraying unit).

A semiconductor wafer 5 that is the subject to be inspected is rested onan inspection table 6, serving also as holding means (holder) or vacuumsuction means (vacuum suction device), and is clamped or held withvacuum. A scanning stage 8 is mounted with an air spindle motor 7 towhich inspection table 6 is attached, and is capable of generallyhorizontal movement backwards and forthwards in an axial direction. Airspindle motor 7 rotates at high speed in the θ direction whilecontrolling inspection table 6 to a prescribed speed, and at the sametime, the scanning stage 8 moves inspection table 6 in the scanningdirection. A detection system 2 and a laser irradiation position 10 arestationary. Semiconductor wafer 5 relatively moves with respect to laserirradiation position 10, so that the area of laser irradiation position10 is cooled by fixedly providing the gas spraying means at an arbitraryposition. The gas spraying means is constructed by eliminating slidingparts to the greatest possible extent, to restrict generation of dustfrom the gas spraying means.

A laser beam 11 irradiated from a laser light source 20 is reflected bya mirror 4 to pass through a projecting portion 3, and is then directedto laser irradiation position 10 on the surface of semiconductor wafer5. With laser irradiation position 10 being fixed, semiconductor wafer 5is moved relatively in the scanning direction while being rotated athigh speed in the θ direction, to detect foreign particles on the entiresurface of semiconductor wafer 5. As laser irradiation position 10 isirradiated with laser beam 11, scattered light is generated from foreignparticles or defects, which scattered light is detected by detectionsystem 2. Gas 9 of high cleanliness is sprayed onto laser irradiationposition 10 of semiconductor wafer 5 from gas spraying unit 1continuously at least during the time in which laser beam 11 isirradiated.

In the example shown in FIG. 1, the gas spraying direction from gasspraying unit 1 to laser irradiation position 10 is opposite to theirradiation direction of the laser beam. The gas spraying direction doesnot affect cooling efficiency of the area of laser irradiation position10, and similar effects can be obtained when it is set to any directionwith respect to the laser irradiation direction. As the gas for cooling,dry air or nitrogen gas, or gas exhibiting high thermal conductivity,such as helium, argon or the like, may be used.

Laser light source 20 and gas spraying unit 1 are controlled by a CPU orprocessing device 21. The output of detection system 2 is subjected toanalog-to-digital conversion, and is input to CPU or processing device21. Rotation angle data of air spindle motor 7 and position data ofscanning stage 8 are also input to CPU or processing device 21. When theoutput of detection system 2 exceeds a predetermined threshold value,CPU or processing device 21 obtains the coordinate on the wafer from therotation angle data of air spindle motor 7 and the position data ofscanning stage 8, and stores the same as defect coordinate data.

FIG. 2 is a schematic diagram showing an air circuit of the gas sprayingunit 1. The air circuit includes pressurizing means (pressurizingdevice) of gas 17, made of a compressor 19 or the like, dehumidifiermeans (dehumidifier device) made of an air dryer (not shown) or thelike, supply path open/close means (supply path open/close device) madeof an air valve 12 or the like, flow rate measuring means (flow meter)made of a purge meter 13, MFM (mass flow meter) or the like, flow ratevariable means (flow rate variable device) made of a speed controller16, needle valve or the like, dust control means (dust control device)arranged in a plurality of stages, made of two or more filters or thelike, and a supply pipe 30 for connecting these elements. The overallsupply of gas 17 is controlled via CPU or processing device 21.

The pressurizing means and the dehumidifier means in the configurationmay be substituted with dry air supplied as the plant utilities on thesemiconductor manufacturing line, or may be provided individually forthe inspection apparatuses. In either case, similar effects can beobtained. In order to use a gas type different from the air or tosuppress disturbance from the plant side, it is preferable to mount themindividually to the inspection apparatuses. When placing greaterimportance on downsizing and manufacturing cost of the inspectionapparatuses, it is preferable to substitute them with the dry air.

As shown in the figure, gas 17 increased to a prescribed pressure bycompressor 19 is controlled to a prescribed dew point by the air dryer,and is supplied through supply pipe 30. Inflow gas 17, of which supplyand shut-off is controlled by valve 12, passes through purge meter 13that measures a flow rate of gas 17. Thereafter, gas 17 is adjusted byspeed controller 16 to the flow rate that would not contaminate thewafer. After passing through a first filter 14 (first dust controlmeans) and a second filter 15 (second dust control means), the gas issprayed onto the surface of semiconductor wafer 5 as sprayed gas 18.Foreign particles (dust) within gas 17 are removed (captured) by firstfilter 14 (first dust control means) and second filter 15 (second dustcontrol means) provided in a plurality of stages, and thus, sprayed gas18 is sprayed as the gas of high cleanliness. It is noted that speedcontroller 16 and purge meter 13 may be configured with flow ratecontrol means (flow rate control device) such as a MFC 31 (mass flowcontroller) or the like, serving as both the flow rate measuring meansand the flow rate variable means, as shown in FIG. 5. This allows gas 17to be supplied at a stable flow rate with accuracy, without beingaffected by variation in pressure of compressor 19. Accordingly, thecooing performance by sprayed gas 18 is stabilized, and thermaldestruction of foreign particles can be suppressed effectively, andthus, coordinate repeatability of the foreign particles improves.

The set value of the flow rate of gas 17, controlled by MFC 31, is setin a freely changeable manner via a setting screen provided on displaymeans (not shown) formed with a CRT or flat panel display, using inputmeans (input unit) 5 (not shown) formed with a keyboard, touch panel,mouse or the like. The input set value is registered to CPU orprocessing device 21. CPU or processing device 21, based on the flowrate set value, controls via MFC 31 such that the prescribed flow rateof gas 17 is maintained in a stable manner.

FIG. 6 shows a flow rate setting screen of the present embodiment. It isconfigured such that the set value of flow rate supplied from gasspraying unit 1 can be input, via an input device 520, to a flow rateset value input space 33 that is displayed on the setting screen 32provided on the display means (display device). The flow rate of gas 17is controlled in the above-described manner, based on the set value.

Although the flow rate of gas 17 is set via input space 33 in thepresent embodiment, not limited to input space 33, it may be set viaanother screen opened with an icon or button provided on the settingscreen. Any screen enabling setting of at least the set value of flowrate of gas 17 may be used.

FIGS. 3A and 3B show the effect of suppressing thermal destruction offoreign particles by the gas spraying means.

FIG. 3A shows a map (distribution of foreign particles) of the wafer inthe state of occurrence of contamination in a spiral fashion (detectionsensitivity: 40 nm or more). It shows that, with the inspection carriedout by a conventional method without spraying of gas, foreign particlesadherent to the wafer are destroyed by laser irradiation and scatteredaround the area, which adhere to the wafer in the shape of bands. Theforeign particles are distributed in a spiral manner from the inner sidetoward the outer side, because the inspection was carried out whilerotating the wafer, causing the scattered foreign particles to moveoutward along the flow of the air that occurs from the inner side towardthe outer side on the wafer surface during inspection due to viscosityof the air.

FIG. 3B shows a map of the wafer in the state of absence of spiralcontamination (detection sensitivity: 40 nm or more). The inspection wasconducted under the conditions exactly the same as in FIG. 3A, exceptthat the gas was sprayed onto the laser irradiation area on the wafer.

Spiral contamination as in FIG. 3A has not occurred, showing asignificant effect of suppressing thermal destruction of foreignparticles by the gas spraying means. It is noted that the inspection wascarried out in the following order: (1) perform inspection of theinitial state, (2) cause foreign particles to adhere, (3) conductmeasurement of FIG. 3B, and (4) conduct measurement of FIG. 3A. That is,FIGS. 3A and 3B are diagrams confirming that the foreign particles aredestroyed in the absence of gas spraying, after confirming presence ofthe gas spraying effect.

FIG. 4 shows the relation between the laser power and the flow rate ofsprayed gas having the effect of suppressing destruction of foreignparticles, obtained through experiments.

The experimental conditions are as follows:

(1) laser wavelength: 532 nm

(2) laser spot diameter: elliptical shape of 50 μm×10 μm

(3) spindle rotational speed: 1500 r/min

(4) stage feed pitch: 20 μm

(5) foreign particle detection sensitivity: 40 nm

(6) inner diameter of gas (air) spraying nozzle: 2.5 mm

(7) evaluated wafer: bare wafer of φ 300 mm

(8) air spraying angle (elevation angle): 23°

The experimental methodology is as follows:

(1) Perform measurement of the wafer in the initial state.

(2) Intentionally cause foreign particles that would readily bedestroyed (e.g., organic matter) to adhere to the wafer surface.

(3) Perform measurement in the state where air is sprayed onto theportion irradiated with laser.

(4) Perform measurement in the state where air is not sprayed onto theportion irradiated with laser.

(5) Determine that there is the air spraying effect and mark “◯” whenspiral contamination does not occur in (3) but occurs in (4).

(6) Determine that there is no air spraying effect and mark “X” whenspiral contamination occurs in (3).

(7) Determine that adherence of foreign particles is insufficient whenno spiral contamination occurs in both (3) and (4), and cause theforeign particles to adhere again and perform measurement of (3) and(4).

(8) Conduct evaluation of (2) through (7) using the air spraying flowrate [l/min] and the laser power [mW] as parameters.

The method for determining the air spraying effect is as follows. Thestate after measurement is compared with the initial state beforeadherence of the foreign particles. It is determined that there is noair spraying effect if there is spiral contamination as shown in FIG. 3Aor the track of destruction of the foreign particles. In contrast, it isdetermined that there is the air spraying effect if there is no spiralcontamination compared with the initial state.

It has been confirmed that the spraying of the air has an effect ofreducing the range of contamination even in the area shown by X wherethe foreign particles are destroyed. This is presumably because the flowof the air on the wafer surface during inspection is disturbed by thesprayed air, making it difficult for them to adhere to the wafer.

It is understood from the result of FIG. 4 that the gas spraying flowrate required to suppress destruction of foreign particles increases asthe laser power increases, and that the flow rate of at least 15 l/minor more is required with the laser power of 500 mW or more that isnecessary for detection of fine foreign particles. It however has beenconfirmed that the gas supplied with the flow rate exceeding 30 l/minwould likely blow off the adhered foreign particles. As such, it isdesirable that the flow rate of gas 17 is set preferably in the range of15 to 30 l/min to guarantee the performance of foreign particlerepeatability. More desirably, using the following expression 1 obtainedfrom the result of FIG. 4, the flow rate is set preferably in the rangeof not lower than the flow rate obtained from the expression 1 and notgreater than 30 l/min:

y=0.015x+7.5  (expression 1)

where y is the spraying flow rate of gas 17 (l/min), and x is the laserpower (mW). When gas of high thermal conductivity, such as helium gas orthe like, is used, a greater effect of suppressing thermal destructioncan be obtained with the less flow rate of gas 17, which can improveboth the detection sensitivity and the foreign particle repeatability.

Further, the relation between the laser power and the spraying flow rateshown in FIG. 4 is displayed as a scatter plot 34 on the setting screenprovided on the display means shown in FIG. 6 or on a screen of databaseseparately provided. By displaying scatter plot 34 as the referencedata, it is possible to prevent a setting error of flow rate of gas 17with respect to the laser power. Furthermore, it is configured to allowan alarm message to be displayed on the display means or theregistration to be rejected, when the flow rate setting of gas 17 out ofthe relation of FIG. 4 with respect to the laser power is input, basedon the expression 1 above and the upper limit of the flow rate.

As described above, according to the present embodiment, gas of highcleanliness is sprayed, which ensures that inspection of the wafer iscarried out while maintaining the conventional cleanliness. The adheredforeign particles are not blown off, which ensures that the conventionalperformance of foreign particle repeatability is maintained. Further,according to the present embodiment, it is possible to increase thedensity of laser energy at the limit of not destroying the foreignparticles. Even in the event that the laser beam is irradiated exceedingthe laser energy density at the limit of not destroying the foreignparticles, the range of scattering of the destroyed foreign particlescan be made smaller than in the conventional case.

Although the embodiments of the present invention have been describedabove, it will be apparent to a person skilled in the art that thepresent invention is not limited thereto, but various changes andmodifications are possible within the spirit of the present inventionand within the scope of the appended claims.

1-3. (canceled)
 4. An optical inspection apparatus comprising: a stagewhich moves a substrate relative to a laser beam; a light irradiationunit which irradiates foreign particles on the substrate with the laserbeam; a light detection unit which detects light scattered from theforeign particles; a gas spraying unit which supplies a gas to saidforeign particles; and at least one of a dust control unit and adehumidifier unit disposed in a gas flowing path of the gas supplyingunit.
 5. An optical inspection apparatus according to claim 4, whereinsaid gas supplying unit includes a gas spraying unit.
 6. An opticalinspection apparatus according to claim 4, wherein said substrate is asemiconductor wafer, a glass substrate or a disk substrate.
 7. Anoptical inspection apparatus according to claim 4, wherein said dustcontrol unit includes a filter.
 8. An optical inspection apparatusaccording to claim 4, wherein said dehumidifier unit includes an airdryer.
 9. An optical inspection apparatus according to claim 4, whereinthe gas is dry air, nitrogen gas, helium or argon.
 10. An opticalinspection apparatus according to claim 5, wherein the gas spraying unitincludes a supply pipe.
 11. An optical inspection apparatus according toclaim 5, wherein the said gas spraying unit includes a supply pathopen/close unit.
 12. An optical inspection apparatus according to claim5, wherein the said gas spraying unit includes at least one of a flowrate measuring unit and a flow rate variable unit.
 13. An opticalinspection apparatus according to claim 5, wherein the said gas sprayingunit sprays gas at least during the time in which the laser beam isirradiated.
 14. An optical inspection apparatus according to claim 5,wherein a dew point of the gas is controlled.
 15. An optical inspectionapparatus according to claim 5, wherein at least one of a flow rate anda flow velocity of the gas is controlled based on an energy density ofthe laser beam.
 16. An optical inspection apparatus according to claim5, further comprising a display unit which is adapted to display arelation between an output power of the laser beam and a flow rate ofthe gas.